The invention relates to a device for compensating the inertia forces in reciprocating piston machines, consisting of a balance shaft housing and of a pair of balance shafts mounted in the latter in friction bearings and having compensating weights, the balance shaft housing being fastened in one part and directly or indirectly to the engine block, and the balance shafts being driven from the crankshaft mounted in its main bearings.
The purpose of balance shafts is to compensate inertia forces and moments of inertia occurring in reciprocating piston machines. They are used preferably in high-speed lightweight internal combustion engines, in particular in pairs in engines with four cylinders in series, for compensating second-order inertia forces. In the latter application, they rotate at double the crankshaft rotational speed, that is to say at up to and above 10,000 revolutions.
This means that they must satisfy extreme requirements with regard to precision and mounting and also lubrication. At the same time, however, they are to be as lightweight as possible, inexpensive to produce and easy to assemble, and, in addition, take up as little construction space as possible in the crankcase. In principle, two different forms of construction are possible: either the balance shaft is produced in one part with its compensating weights or it is xe2x80x9cbuilt upxe2x80x9d, the compensating weights being fastened to the finished shaft.
The first is described, for example, in DE 37 05 346 A and the second in U.S. Pat. No. 4,425,821. The one-part form of construction is highly complicated, requires the highest degree of accuracy and, in the case of shafts with more than two bearings, leads to large bearing diameters which present lubrication problems at the high rotational speeds. For these very reasons, rolling bearings are also ruled out. By contrast, built-up balance shafts have, above all, the advantage of allowing smaller bearing diameters, but care must be taken to ensure that the shaft has sufficient rigidity. Moreover, even where more than two bearings are concerned, one-part housings with bearing bushes all round may be used. It is difficult, however, to achieve the highest possible precision, reliable fastening and sufficient unbalance, along with limited outer dimensions. Thus, for example, in the clamping connection of U.S. Pat. No. 4,425,821, the precision, minimum outside diameter and strength of the connection are questionable.
The aim of the invention, therefore, is to design a built-up balance shaft in such a way that, at minimum production cost and with simple assembly, it satisfies the requirements as regards the highest possible precision of the mounting, along with sufficient lubrication, reliable fastening and minimum outer dimensions.
This is achieved, according to the invention, in that
a) the friction bearings of the balance shafts are arranged, together with the main bearings of the crankshaft, in a plane perpendicular to the crankshaft axis,
b) the friction bearings are cylindrical bores in the balance shaft housing which are machined as bearing surfaces,
c) the balance shafts are cylindrical shafts of essentially constant diameter, on which the compensating weights are fastened individually.
On account of a), the balance shaft housing can be fastened simply and the friction bearings can be supplied with lubricating oil of sufficient quantity along the shortest possible path and equally effectively. On account of b), the friction bearings can be machined as accurately as possible at least possible manufacturing cost and with minimal weakening of the balance shaft housing. The minimal weakening of the housing is necessary so as not to impair the precision of the mounting by the distortion of the housing. Shafts of constant diameter according to c) can be produced particularly inexpensively and accurately. The individually fastened compensating weights make it possible to have small bearing diameters and allow simple assembly, they are simply xe2x80x9cslipped onxe2x80x9d during the introduction of the shaft into the housing and then secured.
If, in a development of the invention, the balance shaft housing consists of light metal and is fastened to the engine block by means of screw bolts, the screw bolts are in each case arranged in pairs in a plane perpendicular to the crankshaft axis through the main bearings (claim 2). By means of this arrangement of the screw bolts, it is possible for the balance shaft housing, consisting of light metal with its higher coefficient of thermal expansion, to be fastened with sufficient precision in an engine block consisting of cast iron. The screwing only in the plane of the main bearings means that the balance shaft housing has a small contact surface on the engine block. It must be remembered that, for this purpose, normal screw bolts are used, not locating bolts. The former allow compensation of the differences in expansion by a slight displacement of the parts in relation to one another which is possible because of the small contact surface. There is therefore no warping of the light-metal housing which would impair the precision of the mounting.
For simplification and to save construction space, it is also within the scope of the invention for the compensating weights to possess at least one axially perpendicular end face which, together with a machined surface of the balance shaft housing, forms a thrust bearing (claim 3). Usually such bearings would be provided on both end faces so that the balance shaft is positioned axially with high precision without additional components.
In a preferred embodiment of the invention, each of the friction bearings is supplied with lubricating oil from the nearest main bearing of the crankshaft (claim 4). Thus, a sufficient oil quantity is apportioned to each bearing along the shortest possible path and therefore with only a small pressure loss. As a result, the bearings do not require a specific supply of lubricating oil through a longitudinal duct which would lead to an uneven distribution of the lubricating oil. This type of oil supply is particularly advantageous when the main bearings are themselves supplied from outside, that is to say not through a longitudinally bored crankshaft.
In a development of this preferred embodiment, the balance shaft housing possesses a first and the main bearing a second vertical lubricating duct which are in alignment with one another and are connected to one another by means of a sleeve (claim 5). The vertical lubricating ducts, of course in relation to the vertical plane of symmetry of the engine, lie in the same plane as the screw bolts and, during assembly, are connected to one another in a very simple way by being slipped onto the sleeve. Moreover, the sleeve can absorb vertical offsets between the balance shaft housing and the engine block. The sleeve can be a very simple tubular piece, because there is no need for particular leak tightness in the crankcase.
In a particularly advantageous arrangement, the first lubricating duct is guided between the friction bearings of the two balance shafts and is connected to the friction bearings by means of tap bores. (claim 6). This arrangement offers the shortest possible connecting paths with low flow resistance, and simple machining.
It is within the scope of the invention, furthermore, for the compensating weight to be a cylindrical ring with two axially perpendicular end faces and with a cutout in the longitudinally central region, so that the compensating weight consists of two ring parts adjoining the two end faces and of a segment part lying between them, and for the compensating weight to be connected firmly to the shaft (claim 7). The edge-side closed rings absorb the tensile force, offer a firm connection with an accurate fit and also stiffen the shaft, and, in the simplest case, they can be shrunk on. The cutout in the longitudinally central region, said cutout extending over virtually half the circumference, allows high mass eccentricity along with a small outside diameter. The segment part surrounds the other half of the circumference. Said segment part bears against the tension zone of the shaft subjected to bending stress and thus considerably increases the rigidity of the latter. Inhibiting deformation in this way by an increase in the moment of resistance is beneficial to the shaft because of reduced bending stresses and to the bearings because of a better carrying profile on account of the lower flexion. The shafts can therefore be produced with a smaller diameter. Thus, overall, along with precise mounting, smaller outer dimensions can be achieved.
If the mass eccentricity which can be achieved in this way is not sufficient, with only a slight increase in the outside diameter the compensating weight can have, on the side facing away from the cutout, a thickening which increases the eccentric mass (claim 8).
In an advantageous design variant, the compensating weight is connected to the balance shaft in the cutout by means of an energy beam weld seam which is made on both sides at the intersection of the cutout plane with the balance shaft (claim 9). A particularly rapid and reliable connection is thus made, which, in the case of weld seams located opposite one another and produced preferably with a laser, is also completely free of distortion.
In another advantageous design variant, the balance shaft has at least one first transverse bore which is in alignment with at least one second transverse bore of the compensating weight, said two bores receiving an essentially cylindrical connecting element (claim 10).